This invention relates to a highly sensitive thermal recording material excellent in response to heat.
Thermal recording materials are generally composed of a support and a thermal recording layer formed thereon composed mainly of a usually colorless or light-colored electron-donative dye precursor and an electron-accepting developer. When they are heated by a thermal head, thermal pin, laser or the like, the dye precursor reacts with the developer in a moment to give recorded images. They are disclosed in Japanese Patent Publication Nos. 4160/68 and 14039/70, etc. Such thermal recording materials are advantageous in that a record can be obtained by means of a relatively simple apparatus, that their maintenance is easy, and that they cause no noise. They are utilized in various fields of recorders for measurement, facsimile, printers, terminals of computer, label, automatic ticket machines for passanger tickets and the like, etc. Particularly in facsimile, there is a greatly growing demand for thermal recording. Moreover, facsimile is being made high-speed for the purpose of reducing transmission cost. With such speedup of facsimile, the thermal recording materials have come to be required to have higher sensitivity.
In high-speed facsimile, a standard manuscript of the A4 size is transmitted and received in several to 20 seconds, and therefore a current flows to a facsimile thermal head repeatedly for a very short time of several milliseconds or less every time, and heat energy produced thereby is transmitted to a thermal recording sheet to carry out an image formation reaction.
In order to carry out the image formation reaction by means of heat energy in such a short time, it is necessary to use a thermal recording material excellent in response to heat. For increasing its heat-reactivity, its compatibility between the developer and the dye precursor should be improved. For this purpose, a sensitizer is used if necessary. Since the sensitizer dissolve or enclose the neighboring dye precursor and developer to promote a color reaction when melted by the transmitted heat energy, improvement of the response to heat of the developer or its compatibility with the dye precursor and the developer is one method for making the thermal recording material highly sensitive.
As such a method, there are disclosed the addition of waxes in Japanese Patent Application Kokai (Laid-Open) No. 19231/73; that of nitrogen-containing compounds, carboxylic acid esters, etc. in Japanese Patent Application Kokai (Laid-Open) Nos. 34842/74, 149353/75, 106746/77, 5636/78, etc.; that of naphthol derivatives in Japanese Patent Application Kokai (Laid-Open ) Nos. 64593/82 and 87094/83; that of naphthoic acid derivatives in Japanese Patent Application Kokai (Laid-Open) 64592/82, 185187/82, 191089/82 and 110289/83; that of benzoic acid ester derivatives in Japanese Patent Application (Kokai) Nos. 148688/82, 182483/82, 112788/83 and 162379/83.
However, thermal recording materials produced by employing these methods are still insufficient in depth of developed color and sensibility of color development.
The present inventors have investigated various sensitizers in order to obtain a highly sensitive thermal recording material more excellent in response to heat.